Graduate member_role branch protection GraphQL fields to stable

What does this MR do and why?

Follow-up to Remove custom_roles_for_protected_branches feat... (!251222 - merged)

The member_role field on branch protection access level types, and the matching member_role_id argument on the corresponding input types, were marked experiment: { milestone: '19.2' } because they were gated behind that flag. Now that the flag is being removed and the underlying feature is generally available, this drops the experiment: markers so the schema reflects that these are stable, and regenerates the GraphQL reference docs and introspection schemas.